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1. Field of the Invention
The present invention relates to denitrification. More particularly, the present invention relates to using low-solubility metal stearates for performing denitrification.
The metal stearate denitrification system is a nutrient removal system in which nitrate is removed from water using a biological treatment method that incorporates a metal stearate such as aluminum stearate for denitrifying bacteria metabolism. In addition, some phosphate removal occurs through the precipitation of metal phosphates such as aluminum phosphate.
2. General Background of the Invention
Brief description of presently used technology and its disadvantages.
Denitrification can be used to remove excess nitrate from waste waters. These include waste waters released as oxidized effluent from home septic tanks, from municipal and agriculture oxidation lagoons, from landfills, from marine waste systems, from industrial systems, and waste waters generated within enclosed circulating systems such as aquariums. For the purpose of discussion of presently used technology, denitrification of waste waters from home septic tanks and oxidation lagoons will be used as examples.
Denitrification utilizes bacteria to reduce nitrate to nitrogen gas which is lost to the atmosphere. Denitrification rates increase with temperature and require the absence of measurable oxygen or anoxic conditions (Manahan, 1994). Different organic substrates have been tested for bacterial metabolism and the denitrification rate varies with the substrate. The most rapid rates occur using water-soluble organic compounds like acetate; however, low solubility substrates are needed for low maintenance systems. Present technology commonly utilizes wood cellulose as the bacteria substrate (Robertson and Cherry, 1994; Robertson and Anderson, 1999; Robertson et al., 2000). The problem is that cellulose-based denitrification is often incomplete and requires time on the order of a day, rather than an hour, for most of the denitrification to occur (Stoessell et al., 2001). Described herein is the use of low solubility metal stearates, in particular, aluminum stearate which bacteria can use much more rapidly than cellulose for denitrification.
At present, home septic systems, lacking a drain field, discharge their effluent into ditches in rural areas and into street curbs in urban areas. In some cases, an aeration system is used within the tank, to convert nitrogen as amunonium to nitrate prior to discharge. However, present technology does not remove nitrogen as either ammonium or nitrate from the effluent. Similar problems exist with oxidation lagoons which generally use air bubbling systems to oxidize organic matter and attempt to convert ammonium to nitrate. If the effluent is passed through a wetlands, prior to being discharged, some or all of the nitrate will be denitrified provided the residence time is of the order of days. Otherwise, the ammonium or nitrate-rich effluent is input into canals and streams.
3. Description of Related Art
The denitrification system described here was developed to remove nitrate from the effluent of aerobic septic tanks. The typical septic tank discharges ammonium-containing effluent which is oxidized to nitrate in an aerobic drain field in the soil (Wilhelm et al. 1994) . The nitrate is then subsequently removed downstream by denitrification within an anaerobic drain field having a carbon-rich substrate (Lampert and Sommer, 1997) . A problem arises in areas having a high water table (near the surface) or a limited lot size (in a town). Drainage fields are not practical under these circumstances and the effluent is usually discharged by pipe directly to a drainage ditch or street curb. Use of an air-bubbling system within the septic tank can substitute for the aerobic drain field, but the discharge still contains nitrogen in the form of nitrate. To minimize the possibility of eutrophication in surface waters, the nitrate needs to be removed prior to the effluent being discharged into a drainage ditch (Connell and Miller, 1984).
(All Incorporated Herein by Reference)
Connell, D. W. and G. J. Miller. 1984. Chemistry and ecotoxicology of pollution. New York, N.Y.: John Wiley and Sons.
Lambert W. and U. Sommer. 1997. Limnoecology: The ecology of lakes and Streams. Oxford, United Kingdom: Oxford University Press, Inc.
Manahan, S. E. 1994. Environmental chemistry. Boca Raton, Fla.: CRC Press, Inc.
Robertson, W. D. and M. R. Anderson. 1999. Nitrogen removal from landfill leachate using an infiltration bed coupled with a denitrification barrier. Ground Water Monitoring and Remediation 19, no. 4: 73-80.
Robertson, W. D. and J. A. Cherry. 1994. In situ denitrification of septic-system nitrate using reactive porous media barriers: Field trials. Ground Water 33, no. 1: 99-11.
Robertson W. D., D. W. Blowes, C. J. Ptacek and J. A. Cherry. 2000. Long-term performance of in situ barriers for nitrate remediation. Ground Water 38, no 5:689-695.
Stoessell, R. K., D. H. Easley and G. P. Yamazaki. (2001) Denitrification and phosphate removal using Al stearate. Ground Water Monitoring and Remediation. 21, no. 2: 89-95.
Wilhelm, S. R., S. L. Schiff and J. A. Cherry. 1994. Biogeochemical evolution of domestic waste water in septic systems: 1. Conceptional model. Ground Water 32, no. 6: 905-916.
The following U.S. Patents are incorporated herein by reference: U.S. Pat. Nos. 6,100,081; 6,077,429; 5,908,555; 5,800,709; 5,755,966; 5,494,581 and are discussed below. These are examples of patents for biotreatment processes for denitrification of waste waters and strains of denitrifying bacteria. However, none of these patents mention, discuss, or specify the use of metal stearates for denitrification as described herein.
The anoxic biotreatment cell of U.S. Pat. No. 5,908,555 is a denitrification cell designed for removing nitrates from mining, milling, and industrial-fluid wastes, incorporating an additional phosphate source and methanol as the carbon source for the bacterial substrate. This is a high-maintenance flow-through cell, requiring continuous addition of water-soluble methanol and phosphate. The biofilter of U.S. Pat. No. 6,100,081 utilizes layers of peat and wood shavings as a carbon source for the bacterial substrate for denitrification and for other processes involving water purification. The biological aerated filter of U.S. Pat. No. 5,800,709 contains both aerobic (upstream) and anaerobic (downstream) sections to accomplish both aerobic decay and anaerobic denitrification within the same vessel, but the patent does not specify a particular carbon source for the bacterial substrate for denitrification. In general, filter systems are not low maintenance, requiring backwashing to remove clogging from bacteria flocculates (biosols) and other particulate matter. Modification of activated sludge systems and bioreactors have also been proposed to facilitate denitrification in systems designed primarily for aerobic decay, e.g., U.S. Pat. Nos. 5,755,966 and 5,494,581. Finally, even the use of strains of bacteria have been patented for denitrification, e.g., a patent (U.S. Pat. No. 6,077,429) has been issued for the anaerobic perlace bacteria to denitrify nitrate and also to breakdown perchlorate.
Common soap contains metal stearate, but the metal stearate used in common soap is soluble in water and would not last long if one tried to use it as a carbon source for denitrifying bacteria.
The unique aspect of the denitrification system of the present invention is the use of insoluble and hydrophobic metal stearate as a bacterial substrate for food for the denitrification process. For an application example, the denitrification system of the present invention can be used in an in-line system for denitrification of aerobic septic-tank effluent prior to being discharged into a drainage ditch or to a street curb.
Flow-through column experiments show aluminum stearate to be an extremely efficient substrate in denitrification, removing up to 26 mg/l of NO3-N in an hour from input solutions containing 36 mg/l NO3-N. Ca stearate was less efficient, removing 13 mg/l of NO3-N in an hour from similar input solutions; however, at low N concentrations, removal rates were similar for both Al and Ca stearate. These results are described in Stoessell, Easley, and Yamazaki, (2001xe2x80x94copy attached to U.S. Provisional Patent Application Serial No. 60/205,158) for a number of different input solution concentrations of nitrate. Cellulose and other organic-aluminum compounds were also tested. These compounds were found to be utilized much less rapidly by bacteria as when using aluminum or calcium stearate. The general insoluble and hydrophobic nature of metal stearate compounds keeps it within a container and stable until bacteria destroy it as part of their metabolism. The use of Al stearate has an additional potential for phosphate removal. As aluminum is released it is precipitated, and some phosphate is removed from the effluent as an aluminum phosphate precipitate or by sorption onto precipitated aluminum hydroxides.
Use of the metal stearate denitrification system of the present invention in a denitrification cell is novel in its use as a flow-through cell for septic tank effluent that requires zero-maintenance over time spans of several or more years. The system withiri the cell eliminates any need to add phosphate because it a common constituent of the effluent. The cell can be a plastic container with a porous liner containing a loosely-packed mixture of metal stearate adhering to inert solids such as porous volcanic scoria with an overall porosity of about 50% (alternatively, though not preferably, another suitable inert solid such as pea gravel could be used). A small amount of soil is used as a bacteria source in the cell. The effluent is fed from an inlet port at the bottom to an outlet port at the top. The liner can be removed and the reactant mixture replaced once the stearate has been eaten by the bacteria. The metal stearate reactant mixture should last from 5 to 20 years, depending on the average daily effluent volume and input dissolved oxygen and nitrate concentrations, the container volume, and the amount of inert material used to keep the permeability and porosity high. A weight ratio of 0.34 of stearate, OOC(CH2)16CH3 to dissolved oxygen, O2, is required to remove the oxygen, and a weight ratio of 0.22 of stearate to nitrate, NO3, is required for complete denitrification (Stoessell et al., 2001). These weight ratios can be used to compute the amount of metal stearate needed in the container for complete denitrification to occur for a particular time period.
The denitrification system example in a denitrification cell can be more completely described as a flow-through plastic container with a replaceable porous liner filled with a porous and permeable mixture of industrial-grade aluminum stearate and volcanic scoria with a small amount of soil as a bacteria source. Oxidized effluent from an aerated home-septic tank enters the container near its base and exits near its top. The insoluble and hydrophobic aluminum stearate remains in place as the fluid flows through the container. The aluminum stearate is gradually destroyed by.bacteria that use the stearate as a metabolic food source. Some of the released aluminum is precipitated as aluminum phosphate. The entering effluent becomes anaerobic as bacteria removed the dissolved oxygen and then nitrate is removed by denitrifying bacteria. The container system is capable of denitrifying up to 26 mg/l of NO3 in a reaction time of one hour using aluminum stearate.
This container system is advantageous in areas having either a high water table or limited lot space where a below-ground drain field is not practical for denitrification. At present, under these conditions, nitrogen-containing effluent is usually discharged to a ditch which can lead to nitrogen accumulation in surface waters, forming algae blooms and producing eutrophication in the water column.
Possible Areas of Commercial Application of the Invention
The system used within a denitrification cell can be an add-on to an aerated home-septic system lacking a drain field. Home septic systems without drain fields are in areas having a high water table, such as southern Louisiana, or with limited space, as on town lots in developing countries which lack municipal sewage systems.
The system also has application for use in aquarium systems to denitrify waters. A denitrification cell could be installed inline within the water circulation system of an aquarium to remove nitrate, using a metal stearate as a bacterial substrate.
The system can also be used as within a porous barrier to treat oxidized effluent from oxidation lagoons, landfills, industrial sources, and other aqueous nitrate sources. A mixture of the metal stearate and an inert porous material such as volcanic scoria would be installed as a porous denitrification barrier through which the effluent would be discharged.